[Bug c++/104386] New: no_unique_address causes invalid member alignment of pod struct

[Bug c++/104386] New: no_unique_address causes invalid member alignment of pod struct

[gcc at ebasoft dot com.pl]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=104386

            Bug ID: 104386
           Summary: no_unique_address causes invalid member alignment of
                    pod struct
           Product: gcc
           Version: 11.2.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: gcc at ebasoft dot com.pl
  Target Milestone: ---

struct base
{ [[no_unique_address]]
 uint32_t x; std::byte v;
 };
struct foo : public base
 { std::byte z; };

gcc sizeof(foo) == 8 https://godbolt.org/z/G4Mo3PdKT

clang sizeof(foo) == 12 https://godbolt.org/z/bdzvaMn9c

As I was told it is gcc bug 
https://www.reddit.com/r/cpp/comments/sjx2mk/no_unique_addres_where_can_in_c_standard_instead/

[orbital1337]"Your class base is a POD but its not a POD for the purpose of
layout (since it has potentially overlapping data members). Thus the Itanium
ABI specifies that it's size without padding (dsize) should be set to its size
with padding (sizeof). The first datamember of your class foo is put at
dsize(base). clang does the right thing and puts it at an offset of 8 bytes
whereas gcc ignores that one line of the specification and puts it at an offset
of 5 bytes instead."

[Bug c++/104386] no_unique_address causes invalid member alignment of pod struct

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=104386

--- Comment #1 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
Hmm:
https://github.com/itanium-cxx-abi/cxx-abi/issues/108

[Bug c++/104386] no_unique_address causes invalid member alignment of pod struct

[gcc at ebasoft dot com.pl]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=104386

--- Comment #2 from Artur Bać <gcc at ebasoft dot com.pl> ---
I think it is connected with this case

https://godbolt.org/z/cvfs1KqGW
https://godbolt.org/z/vdzTzo7be

//sizeof(foo) ==12
struct base
{
uint32_t x{};
std::byte v{};

base() noexcept = default;
};

struct foo : public base
{
std::byte z;
};

//sizeof(foo) == 8
struct base
{
uint32_t x;
std::byte v;

base() noexcept : x{}, v{} {}
};

struct foo : public base
{
std::byte z;
};

and

https://www.reddit.com/r/cpp/comments/slfugx/comment/hvs4n7j/?utm_source=share&utm_medium=web2x&context=3

"Som1Lse · 5 hr. ago

Interestingly GCC disagrees with itself:

    -std=c++17 it considers base to be a POD and doesn't reuse padding.
    -std=c++20 it considers base to not be a POD and agrees with Clang, reusing
the padding bytes."

[Bug c++/104386] no_unique_address causes invalid member alignment of pod struct

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=104386

Andrew Pinski <pinskia at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Severity|normal                      |blocker

[Bug c++/104386] no_unique_address causes invalid member alignment of pod struct

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=104386

Jonathan Wakely <redi at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
     Ever confirmed|0                           |1
             Status|UNCONFIRMED                 |NEW
   Last reconfirmed|                            |2022-03-29